The geothermal energy is a renewable clean energy which is green, low-carbon, and recyclable. With the advantages of being reserved in a large scale, widely distributed, clean and environment-friendly, stable and reliable, etc., the geothermal energy is a clean energy which is practical, feasible and competitive. China has rich geothermal resources, and as shown in the data publicized by the Ministry of Land and Resources in 2015, the quantity of geothermal resources worldwide within a burial depth of 5 km is about 4,900 trillion tons of standard coals, and about ⅙ of them are reserved in China. The reserve of the middle-deep-stratum geothermal resources with a burial depth of 200 to 4,000 m is about 1,370 billion tons of standard coals, and the reserve of the dry-hot-rock geothermal resources with a burial depth of 3 to 10 km reaches 860 trillion tons of standard coals; thus the development and utilization potential is enormous. Nonetheless, in China the proved geothermal resources are mainly hydrothermal, and the large-scale high-temperature geothermal fields are few; the medium-low temperature geothermal system has complex geothermal reservoir conditions, a low water temperature, a small flow rate, and a poor quality; with the current geothermal development modes, the production is low, the heat recovery power is limited, and the recharging is difficult; thus the development and utilization of the hydrothermal geothermal resources in China are greatly restricted.
Currently, there are mainly two types of development modes for hydrothermal geothermal fields. The first type is to recover heat without water, including using a buried pipe that recovers heat through heat conduction, and using a single-well downhole heat exchanger that performs a convective heat exchange. The second type is to perform geothermal exploitation by directly exploiting the geothermal fluid.
The mode that recovers heat without water is mainly adopted for the shallow-stratum medium-low temperature geothermal development without exploiting the geothermal fluid, which avoids the difficulty of recharging, saves the procedures such as de-gritting and anticorrosion, and achieves the advantages such as environment protection and simple process. However, the heat recovery power is limited, and the exploited geothermal energy can only be used for the direct application modes such as the heat supply, rather than the geothermal energy power generation. Thus, the energy utilization rate is low, and this mode is not suitable for the deep-stratum high-temperature geothermal development.
Directly exploiting a geothermal fluid is a geothermal development method with the highest heat recovery power. Nevertheless, in China, there are a large number of geothermal reservoirs of complex geological conditions, and their permeabilities are low; thus the method has a low production per well, and the recharging is difficult; therefore, in some geothermal fields, the recharge rate is low and the underground water level drops. If a reservoir reformation is performed with a hydraulic fracturing method, the geothermal development cost will be obviously increases, and the underground water resources will be polluted.
In conclusion, the prior art has the following problems: regarding the complex geothermal reservoir conditions in China, the current geothermal development modes have a low production per well, and the recharging is difficult.